Transition for waveguide



July 7, 1959 D. A. LANclANl 2,894,213

TRANSITION FOR WAVEGUIDE Filed Jan. 5, 1955 ATTORNEYS United States Patent O TRANSITION FOR WAVEGUIDE Daniel LA. Lanciani, West Medford, Mass., assignor to Microwave Associates, Inc., Boston, Mass., a corporation of Massachusetts vApplication January 3, y1955, Serial No. 479,285 y 9 Claims. (Cl. 33321) This invention relates to a parallelepipedal to cylindrical waveguide transition and in particular to an endfeed transition for circular waveguides propagating the TEM mode.

Various methods have been proposed for establishing the TEM mode in a circular waveguide. Generally these methods have contemplated the insertion of power in a number of points in a circular waveguide, and where a circular waveguide is being fed from a rectangular guide, it is conventional to divide the power into a series of separate waveguide structures which in turn feed the circular waveguide. The feed into the circular waveguide has been in the form` of slots, either parallel to the axis of the circular waveguide or radial to the end plate. However, in al1 of these cases the division of power into a plurality of separate waveguide structures has required bulky and expensive waveguide plumbing. Not only has the structure been awkward but it has very often Aresulted in the propagation of undesired modes, particularly the TE21 mode, which can propagate in a smaller size circular waveguide than the TEM mode.

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long sides of both the rectangular guide and the two transition chambers. The axes of the circular and rectangular guides are indicated at c and r, respectively. The two chambers 14 and 16 terminate in a conducting end plate 18 which is so located as to appropriately tune the two chambers to give optimum radio frequency matching between the rectangular waveguide and the slot array at the transition design center frequency.

The chambers are preferably coupled to the circular waveguide by means of four slots, 22, 24, 26 and 28 in the end plate 21 of the circular lguide 20. In order for these slots to meet the requirements for TE, mode propagation in the rectangular guides and still satisfy the TEM mode propagation in the circular guide,`the following conditions must be satisfied: The slots should be mounted radially in the circular waveguide end plate 21 so that they intercept the maximum endl plate current for the TEM mode of the circular waveguide. In addition, with the present circular guide the slots are slanted 45 degrees with respect to a line parallel to the rectangular waveguide axis r and are with the midpcints of the pair of slots on each side of the rectangular waveguide axis spaced either a single one-half guide wavelength at the design center frequency of the chambers apart or an odd multiple of that half guide wavelengths apart along the y rectangular waveguide axis. In other words in the pre- Also such structures have usually exhibited relatively narrow band widths.

It is the object of this invention to provide a relatively simple and compact apparatus for launching the TEM mode in a round waveguide.

A further object of this invention is to provide greatly simplied coupling between a circular waveguide `and a rectangular waveguide which will have a relatively wide band width and a high degree of mode purity.

It is a feature of this invention that it achieves the necessary power division and phase relationships by way of resonant slots feeding the end of the circular waveguide without the use of cumbersome separate waveguide structures for each feed.

This invention may be more readily understood by reference to the drawings in which Fig. 1 is a view along the axis of the circular guide toward the junction;

Fig. 2 is a sectional side view of the apparatus according to Fig. 1 along the line Z-Z of Fig. 1; and

Fig. 3 is a section taken along the line 3--3 of Fig. 2 looking toward the slots leading to the circular guide and showing the internal relationship of the slots in the end ofthe circular waveguide and the E plane power divider in the rectangular waveguide.

Fig. 4 is an end elevation of Fig. 1.

Referring to the gures it will be seen that this invention contemplates the use of a cylindrical, here circular waveguide 20 cEor propagating the TEM mode coupled by means of the transition to the parallelepipedal, here rectangular ANvavegude structure 10 in which the TEM, mode is being propagated. As shown in Figs. 1 and 3, the preferred embodiment of the transition which achieves this coupling is formed of two chambers 14 and 16 separated by an E plane dividing partition, septum, or power divider 12. This partition is in the plane of intersection of theaxes of the` two waveguides and parallel to the ferred embodiment the axes of the slots intersect in, and are inclined 45 to a plane through the intersecting axes of both waveguides, and the distances between the midpoints of pairs of adjacent slots are an odd multiple half guide wavelength at the center frequency for which the chambers are designed. It will be evident that the midpoint of any radial slot is spaced one-half of an odd multiple half guide wavelength from the plane through the circular vguide axis, atright angles to the rectangu lar guide axis. As indicated by the solid arrows in Figs. 1 and 3 showing the electric field vectors in the E plane, this half wavelength spacing and 45 degree slanting pro vide the necessary phase reversal and field orientation which are needed to establish the circular electrical eld in the cylindrical waveguide.

The end plate 18 which tunes the two :resonant chambers is also appropriately located with respect to the centers of the slots. However, if this shorting plate is as close as one-quarter guide wavelength from the nearest pairs of slots 22 and 28 this proximity will tend to cause local field interference and for practical purposes, the shorting plate is therefore located at least three-quarter guide wavelengths. from the centers of the nearest slot pair. Since the two slots in each resonant chamber are separated by a half guide wavelength in most instances, the shorting plate will then be approximately one guide wavelength from the axis c of the circular guide, which is midway between the slots. Slots 22 and 28 will be approximately 3%; guide wavelengths from the shorting plate at the center frequency of the transition and slots 24 and 26 will be approximately one and one-quarter guide wavelengths `from the shorting plate. It would be possible to move the shorting plate a much greater distance from the slots, but the tendency in this event would be to decrease the band width because of the increased frequency sensitivity of a longer line length.

The cross section of the two chambers 14 and 16 is dependent on the center frequency of operation of the transition since a number of transitions would be necessary to cover the entire band width which may be transmitted through a single size of standard rectangular waveguide. Assuming that the H-plane dimensions of the chambers 14, 16 diter from that of the waveguide 10 with which the transition is to be used,I it is necessary rst to employ an H-plane transformer 25 as shown in Figs. 2 and 3 in order to match the standard rectangularwaveguide to the transition. Such a transformer is simply a tapered section which decreases, or as the case may be, increases the H-plane dimension of the guide as clearly shown in Pig. 2.

Since the transition utilizes an E-plane power divider it will be necessary in all cases to utilize an E-plane tapered section or stepped transformer 28 which, together with the power dividing partition 12, will transfer power from the TEU, mode in the rectangular guide tothe same mode in each of the two chambers of the transition. To avoid the formationof unwanted reflections, is is desirable that this transformer section be at least one guide wavelength long and that the power divider be similarly tapered as shown in Fig. 3 to form a sharp edge projecting into the rectangular waveguide. The distance fromthe edge of the E-plane power divider which forms Ythe beginning ofthe tapered or stepped transformer section 'toitsintersection with the axis c of the circular waveguide will therefore be on the order of two and one-quarter-free space wavelengths, assuming that the chamber should have at least one wavelength between the end of the transformer section and the intersection with the axis c of the circular guide.

It is well known that a plurality of identical slots spaced one half guide wavelength along the edge of a waveguide section will each` couple. out an equal amount of energy from the chamber. ln the above described construction the power in the rectangular waveguide is dividedequally between the two chambers 14 and 16 by means of the power divider 1.2, and because there are two slots in each chamber spaced one half guide wavelength apart, one fourth of the total power in the original rectangular waveguide will be coupled through each slot to the circular waveguide `which projects at right angles from the transition. In addition, as mentioned above, the odd multiple of half guide-wavelengths spacing between the two slots in each chamber results in the necessary phase reversal `in o rder to couple the energy tothe TEM mode in the circular guide.

The shape and size of these` slots is important in determining the band width and characteristic of the coupling. For example, in order to minimize `the problem of breakdown across theresonant slot and increase the power handling capacity it is desirable to shape `the slot so as to lengthen the radio frequency `breakdown path. This can Vbe done by increasing the width of the slot in the center where the maximum energy is transmitted.

The fact that the slots are mounted radially in the end plate 21 tends to reduce coupling to the undesired TE21 mode in the circular waveguide. Increasing the thickness of the end plate also has this eifect; however, this thickness cannot be made very large without upsetting the slot coupling to the desired TEM mode.

The width and length of the slot also require a compromise between power handling capacity and mode purity. 4Increasing the width and at the same time maintaining a resonant condition by adjusting the slot length is feasible so long as the width does not approach a point at which the TEzl mode becomes excited in the circular waveguide. A slot width slightly less than onequarter wavelength and a slot length slightly greater than one-half wavelength has proven optimum.

The transition slot structure described permits direct coupling between the TEN mode in the rectangular waveguide chambers and the TEM mode in the circular waveguide. If no lslots were present in the resonant chambers the TEM electric field vectors would be parallel with the shorting end plate, but the presence of the resonant slots distorts the iield so that the vector of maximum electric ield lies directly across the narrow width of the slot. As a result, coupling through any slot radial ;to,the.circular guide inthe narrow side of these chambers will-tend tobev parallel with the circular electric 4field of the TEM modein Vthe circular guide. This four Slot..farranaelnent,radiatingV equal energy, as described, provides excellent construction.

In the apparatus as shown in Figs. 1 and 3 the two resonant chambers side by side have a total Width of somewhat less than the diameter of the circular waveguide and, in fact, tend to cover portions of the outer edges of each slot. This eifect is in some instances desirable. For some wavelengths it may be desirable to separate the chambers, the method of feed being still through a tapered or stepped transformer.

The use of the E-plane power dividing partition 12 is essential for satisfactory mode purity and band width but not necessarily for the physical operation of this device at frequencies where the two chambers can be adjacent. If this power dividerv 12 is eliminated coupling will still take place through the radial slots to the circular waveguide. Unfortunately, the single cavity will then have an oversized E plane dimension and a result will be the excitation of higher order modes in the rectangular waveguide section, which in turn will give rise to unwated TEM and possibly `'PE21 modes in the circular waveguide. The band width will yalso tend to be ydecreased if the power divider is not used. In general, these effects will appear whenever the E-planendimension in the resonant chamber becomes larger than `the H-plane dimension, and Vof course for lower frequencies a separation of the chambers may be essential.

The method of feeding the circular waveguide vfrom two small chambers through a slotted end plate effectively rejects the lower undesired modes, particularly the TE21 mode, as well as certain of the higher modes. The suppression ofhigher modes may be aided by the of a circular waveguide which is too small to permit the higher modes such asV the TE and TE31 to be successfully propagated. Where this technique results in an initial circular waveguide section which is not sufficiently large to utilize the low loss properties of an oversize TEMV mode circular waveguide, low loss transmission is obtained by tapering to a larger diameterbeyond the transition section@ It has been found `that for a transition of this configuration the band width vfor a maximum VSWR of 1.2 to 1 is approximately 7% and the mode purity over this range exceeds 15 db or 97%. The total insertion loss of this transition is less than 1A db over the same band width.

While this invention has been described in respect to a single embodiment, it is obvious that the basic concepts can be implemented in a number of configurations without departing from the spirit of the invention which is encompassed in the following claims. i

I claim:

l. A microwave transition for coupling parallelepipedal and cylindrical waveguides, which comprises: a parallelepipedal waveguide; at the end of said parallelepipedal guide a power dividing partition parallel to a pair of walls of the guide, for establishing two resonant chambers; a cylindrical waveguide ending at a wall comrnon to bothchambers, with the longitudinal axis of the cylindrical guide in the plane of said partition and at right angles to the longitudinal axis of thel parallelepipedal guide; and a plurality of slots in said common wall, said slots being radially inclined substantially 45 to the plane through said guide axes, and with the midpoints of pairs of adjacent slots spaced ank odd multiple half guide wavelength at the design center frequency `of said chambers, for directly coupling the ending of the cylindrical guide to the chambers.

2.V Waveguide apparatus for coupling a rectangular waveguide and a 4circular waveguide, which comprises: `a rectangular waveguide; two adjacent resonant chambers at the end of the rectangular guide; a tuned shorting plate for each of said chambers; a circular waveguide mounted on said chambers with its axis perpendicular to the axis of the rectangular guide; an end plate separating the circular guide and-the chambers; andfinsaid endrplate a performance in a very simple plurality of radial slots inclined substantially 45 to the plane through said guide axes, and with the midpoints of pairs of adjacent slots spaced an odd multiple half guide wavelength at the design Acenter frequency of said chambers, for directly coupling the `circular and rectangular guides.

3. Waveguide apparatus for coupling the TEN mode in a rectangular waveguide to the TEO, mode in a circular waveguide, which comprises: a rectangular waveguide; a partition including the axis of the rectangular waveguide for `dividing its end into two parallel rectangular chambers; a .circular waveguide mounted with its axis in the plane of said partition and joining at said partition a common wall of said chambers perpendicular to the axis of the rectangular guide; four resonant slots in said wall which separates the circular and rectangular guides, said slots being radially inclined substantially 45 to the plane through said guide axes, and with the midpoints of pairs of adjacent slots spaced an odd multiple half guide wavelength `at the design center frequency of said chambers; and a conductive plate at the end of each chamber at an odd multiple of one quarter guide wavelengths from the midpoint of the nearest resonant slot.

4. A microwave transition for coupling a rectangular and a circular waveguide, which comprises: a rectangular waveguide; a plurality of parallel resonant chambers at the end of said rectangular guide placed with their longitudinal axes parallel to the axis of the rectangular guide and having a common wall that is parallel to said axes; a circular waveguide placed at said wall with its axis at right angles to the axes of the chambers; at least one slot radial to the axis of the circular waveguide in the wall of each chamber separating the chamber from the end of the circular waveguide, the slot inclination to the plane through said waveguide axes being substantially 45, with the slot midpoint spaced from the plane through the circular waveguide axis at right angles to the rectangular waveguide axis one half of an odd multiple half guide-wavelength at the design center frequency of said chambers, such that the direction of the electrical lield across the slot is substantially in registration with the electrical ield of the TEO, mode in the circular guide; and coupling means for propagating a proportion of the energy in the rectangular guide from the end of the guide into the end of each chamber.

5. A microwave transition as in claim 4 wherein the power division between said parallel resonant chambers is such that each slot radiates an equal amount of energy into the circular guide.

6. A microwave transition for coupling a rectangular to a circular waveguide, comprising: rectangular wave guide means; at least one resonant rectangular chamber means opening into and fed by said guide means and having straight surfaces that are throughout substantially parallel to the surfaces of said rectangular guide means; a circular waveguide means extending with an end perpendicular to a wall in the E plane of said chamber means and having a diameter substantially on the order of the width of said Wall of the chamber means such as to reject selected modes higher than the TEO, mode; and a plurality of slots in said wall of the chamber means leading to said circular guide means, said slots being substantially radial to the axis of the circular guide means, being inclined substantially 45 to the plane through the axes of said rectangular and circular guide means respectively, and being with the midpoints of pairs of adjacent slots spaced approximately an odd multiple half guide wavelength at the design center frequency of said chamber means.

7. Transition according to claim 6 wherein the E plane dimension of the chamber means is greater than that of the rectangular guide means, further comprising a transformer means leading from the rectangular guide means to the chamber means.

8. Transition according to claim 6 wherein the H plane dimension of the chamber means is greater than that of the rectangular guide means, further comprising a transformer means leading from the rectangular guide means to the chamber means.

9. A microwave transition for coupling a rectangular to a circular waveguide, comprising: a rectangular waveguide; a pair of rectangular chambers opening into and fed by said guide, said chambers having straight surfaces and axes that are throughout substantially parallel to the axis and the surfaces of said rectangular guide, and being tuned to a chosen center frequency; a circular wave guide at right angles to said chambers and having a diameter substantially on the order of the width of the wall in the E plane of said chambers such as to reject selected modes higher than the TEO, mode; an end plate for said circular guide forming a portion of one side of `each chamber; and two pairs of slots in said end plate one pair leading to each chamber, the slots in each chamber being radially inclined substantially 45 to a plane through the axes of both waveguides, and with their midpoints approximately spaced an odd multiple half guidewavelength at said center frequency.

References Cited in the le of this patent UNITED STATES PATENTS 2,453,760 Schelleng Nov. 16, 1948 2,455,158 Bradley Nov. 30, 1948 2,471,021 Bradley May 24, 1949 2,560,353 Kerwien June 10, 1951 2,643,298 Arnold June 23, 1953 2,764,740 Pratt Sept. 25, 1956 

